From the Cover: Magmatic thickening of crust in non–plate tectonic settings initiated the subaerial rise of Earth’s first continents 3.3 to 3.2 billion years ago

When and how Earth''s earliest continents—the cratons—first emerged above the oceans (i.e., emersion) remain uncertain. Here, we analyze a craton-wide record of Paleo-to-Mesoarchean granitoid magmatism and terrestrial to shallow-marine sedimentation preserved in the Singhbhum Craton (India) and combine the results with isostatic modeling to examine the timing and mechanism of one of the earliest episodes of large-scale continental emersion on Earth. Detrital zircon U-Pb(-Hf) data constrain the timing of terrestrial to shallow-marine sedimentation on the Singhbhum Craton, which resolves the timing of craton-wide emersion. Time-integrated petrogenetic modeling of the granitoids quantifies the progressive changes in the cratonic crustal thickness and composition and the pressure–temperature conditions of granitoid magmatism, which elucidates the underlying mechanism and tectonic setting of emersion. The results show that the entire Singhbhum Craton became subaerial ∼3.3 to 3.2 billion years ago (Ga) due to progressive crustal maturation and thickening driven by voluminous granitoid magmatism within a plateau-like setting. A similar sedimentary–magmatic evolution also accompanied the early (>3 Ga) emersion of other cratons (e.g., Kaapvaal Craton). Therefore, we propose that the emersion of Earth’s earliest continents began during the late Paleoarchean to early Mesoarchean and was driven by the isostatic rise of their magmatically thickened (∼50 km thick), buoyant, silica-rich crust. The inferred plateau-like tectonic settings suggest that subduction collision–driven compressional orogenesis was not essential in driving continental emersion, at least before the Neoarchean. We further surmise that this early emersion of cratons could be responsible for the transient and localized episodes of atmospheric–oceanic oxygenation (O₂-whiffs) and glaciation on Archean Earth.

The emergence of continental crust above sea level (called continental emersion) critically influences atmospheric and ocean chemistry, climate, and the supply of nutrients to the oceans via weathering and fluvial runoff (1, 2). However, it remains unclear when large areas of subaerial continental crust first appeared on Earth (1–13). A rapid and extensive emersion of continental crust at the Archean–Proterozoic transition (2.5 billion years ago [Ga]) is inferred from abrupt shifts in the oxygen isotope compositions of shales and magmatic zircons, zinc isotope composition of iron formations, and an increase in subaerial continental volcanism at that time (1, 3–5, 7). However, >3.0 to 2.7-Ga-old paleosols (ancient horizons of subaerial weathering) and terrestrial sedimentary rocks that formed atop Earth''s earliest stable continental nuclei, the cratons (14–17), provide direct evidence for earlier episodes of continental emersion. This inference is further corroborated by an increase in the diversity of detrital zircon ages in clastic sedimentary rocks from ∼2.8 Ga onwards, representing the development of regionally extensive watersheds at that time (13). Thus, subaerial exposure of continental crust before 2.5 Ga seems evident. However, the exact timing and spatial extent of these emersion events are poorly constrained, and their global significance remains unclear. Moreover, the mechanisms and tectonic settings that drove continental emersion during the Archean also remain ambiguous. A uniformitarian view posits that Archean continental emersion (whether at ∼2.5 Ga or earlier) was driven by plate tectonics (1, 7, 9) with subduction-collision processes forming thick continental crust with high-standing topography via magmatism and compressional deformation, as is observed on modern Earth (2). However, the operation of plate tectonics in the Archean is disputed (9, 10, 18, 19), and a growing body of evidence suggests that subduction-collision processes were not globally prevalent until ∼2.5 to 2.0 Ga (10, 20–25), warranting the consideration of alternative mechanisms for producing subaerial continental landmasses on early Earth.Here, we integrate the Paleoarchean (3.6 to 3.2 Ga) to Mesoarchean (3.2 to 2.8 Ga) magmatic and sedimentary records of the Singhbhum Craton of India to elucidate the timing and underlying geodynamics of craton-wide emersion of continental crust in the Archean. This craton is ideal for studying Archean continental emersion as it hosts widespread Mesoarchean terrestrial to shallow-marine siliciclastic strata (26–29) and one of the oldest paleosols on Earth (the ∼3.29- to 3.08-Ga Keonjhar paleosol) (30) (Fig. 1A), providing an unambiguous record of early subaerial continental crust. We first synthesize detrital zircon data (SI Appendix, Methods and Datasets S1 and S2) from these Mesoarchean strata to determine the timing of emersion of the Singhbhum Craton. Then, we analyze the published compositional data of the craton’s Paleo-to-Mesoarchean granitoids (SI Appendix, Methods and Dataset S3) to reconstruct the history of crustal thickening and chemical maturation before and during the emersion. This allows us to link the physicochemical evolution of Archean cratonic crust to its emersion as the long-term topography of subaerial continents is critically controlled by their thick, silica-rich (less-dense) crust, which experiences large positive buoyancy and thereby a greater isostatic uplift relative to the surrounding thin and mafic (more-dense) oceanic crust (2). In particular, we determine the pressure–temperature (P-T) conditions of formation of the tonalite–trondhjemite–granodiorite (TTG) suite of granitoids—the principal crustal component of the Singhbhum Craton. The P-T data provide a time-integrated estimate of crustal thicknesses and elucidate the tectonic process controlling the craton’s emersion. These crustal thickness values are cross checked against the independent thickness estimates provided by the La-Yb systematics of the TTGs. Finally, a link between crustal thickening, maturation, and emersion is demonstrated via isostatic modeling.Open in a separate windowFig. 1.Spatial distribution and detrital zircon U-Pb ages of the Singhbhum cover sequence. (A) Simplified geological map of the Singhbhum Craton (29, 31) showing the outliers of the Singhbhum cover sequence and their granite–greenstone basement (SI Appendix, SI Text). The orange area in the Inset shows the location of the Singhbhum Craton within the Indian Peninsula. The younger (∼3.0 to 2.8 Ga) granitoids (including those of the Rengali Province) that intruded the outliers of the cover sequence are also shown. The formations comprising the outliers include: Mahagiri (Mhg), Pallahara-Mankaharchua (PM), Simlipal (Smp), Keonjhar (Kj), Birtola (Bir), Achu (Ac), Bisrampur (Brm), and Dhanjori (Dj). (B) Kernel density estimate (KDE) of <±10% discordant detrital zircon (²⁰⁷Pb/²⁰⁶Pb) ages from different outliers of the cover sequence (SI Appendix, Fig. S1). For each outlier, the white arrow shows the weighted mean ²⁰⁷Pb/²⁰⁶Pb age of the youngest detrital zircon population, which represents its maximum depositional age (SI Appendix, Fig. S1). The minimum depositional age (dashed gray line) of ∼2.94 Ga is constrained from a metamorphic event that affected the outliers. The colored bands show the age brackets of the different phases of granitoid magmatism and greenstone belt formation. Data are in Dataset S1. Refer to SI Appendix, Methods and SI Text for details.